Passive location of moving targets using reflections on a target of a non-cooperative transmission follows a known principle based on bistatic operation of the location device. Said bistatic mode of operation assumes in particular the use of at least two reception paths, one path termed the reference path, receiving the signal transmitted by the non-cooperative source along a direct route, and a target path, intended for the reception of echoes reflected by elements located in the space covered by the transmitting source.
Passive location in bistatic mode is facilitated if the position of the transmitting source is known. This is why such systems generally use civil transmission infrastructures intended for example for national or regional radio broadcasting, such as FM radio transmissions intended for the general public. Said infrastructures generally further offer the advantage of uniformly covering a vast territory with fixed installations. In a known way, the techniques of passive location of moving targets using reflections on these targets of a non-cooperative transmission with a “random” spectrum such as for example FM transmissions, generally implement the following operations:                Calculation of the intercorrelation function between the reference signal originating from the direct route of the wave transmitted towards the receiver, and the signal received by the location system after reflection by the target. To do this, the reference signal successively undergoes a series of shifts corresponding to assumptions of time and frequency shifting of the signal corresponding to assumptions regarding the position and velocity of the target that has reflected the signal transmitted by the source.        Detection of intercorrelation “peaks”, said detection being for example carried out by comparison with a threshold.        Estimation of bistatic distance and Doppler velocity by extracting the position of the detected peaks.        
It should be reminded here that the bistatic distance is a distance that can be used to locate the target that has reflected the transmitted signal, on an ellipsoid centred on the transmitting source and the detection system.
However, the implementation of said techniques known from prior art places certain limitations, associated in particular with the nature of the transmissions used. The characteristics of the type of signal transmitted actually have a decisive influence on the detection performance that may be expected from the passive system using the transmitted non-cooperative signal.
In the particular case of FM radio broadcasting transmissions, particular account must be taken of the breakdown of the complete FM band, which typically extends from 87.5 MHz to 108 MHz, in a certain number of separate channels, each channel thus having a limited frequency band, of the order of 200 kHz for example. The use of a transmission channel therefore limits the analysis band of the signal received to a few hundred kilohertz.
In the case of an FM transmission, account must also be taken of the diversity of the transmitted data constituting the signal modulating the carrier. Said data range from simple speech to digital data, possibly coded and multiplexed. The type of modulating signal directly conditions the spectrum width of the signal transmitted within the same channel. Thus the transmitted signal may occupy a frequency band varying from a few hundred Hertz to the whole bandwidth allocated to the channel (typically 200 kHz), for example.
As far as electromagnetic location systems are concerned, it is known that distance resolution is conditioned by the spectrum width of the signal used. Thus, in the case of passive systems using non-cooperative FM transmissions transmitted on a given channel, the distance resolution is limited to several hundred metres, taking into account the maximum width of the FM channel (typically 100 kHz).
Furthermore, it is also known that the contrast between 2 targets or between a target and ambient clutter is limited by the BT product of the signal's band-width B by the integration time T of the measurement made on the signal. Taking into account the band-width of the signal used, which is limited by the channel width and the maximum period of observation, determined by the dynamic characteristics of the type of target sought, we find that this dynamic range is reduced to a value of the order of 40 to 50 decibels. This value considerably limits the dynamic range of the detectable targets, in terms of radar cross section or RCS and detection range, and may induce heavy constraints on the relative geometric arrangement of the transmitter and receiver.
The band-width limitation of the signal transmitted by non-cooperative sources of the FM radio broadcasting station type therefore forms a limitation on the implementation of passive detection systems using the FM band.